Thermoplastic resins such as the polyamides, the thermoplastic polyesters, the polyphenylene ether resins, the polyarylene sulfide resins, the polysulfones and the like are, of course, well known in the prior art. As is also well known in the prior art, these thermoplastic resins generally exhibit a combination of properties including excellent mechanical properties, good high heat resistance and good durability making them particularly useful for the preparation of molded, cast and extruded mechanical and electrical parts. These resins do not, however, lend themselves to utility in those areas requiring good impact resistance since, generally, they exhibit poor impact resistance.
In light of the several particularly good properties associated with the aforementioned thermoplastic resins, considerable effort has been expended in attempts to improve the impact resistance of the various thermoplastic resins without impairing the other desirable properties thereof. In general, these attempts involve the incorporation of a low modulus rubber into a composition comprising the thermoplastic resin. It is, of course, important that the low modulus rubber not separate from the thermoplastic resin during thermoplastic processing operations. It is, then, important that the low modulus rubber either be compatible with the thermoplastic resin or contain one or more reactive groups which will either chemically or physically bond the low modulus rubber to the thermoplastic resin thereby preventing subsequent separation of the two polymers.
As a practical matter, it is, at best, difficult, if not impossible, to find low modulus rubbers that are compatible with a broad range of thermoplastic resins. As a result, most of the efforts to solve the impact resistance problems of the aforementioned thermoplastic resins has been made with elastomers that are either directly prepared or are modified to contain one or more functional groups that will, in some way, bond with one or more groups contained in the thermoplastic resin used in the composition.
An early attempt to produce thermoplastic resin compositions having improved impact resistance with random type elastomers, which attempt has had some degree of commercial success, at least where a polymer modified with an acid or acid derivative is used as the impact modifier, is taught in U.S. Pat. No. 4,172,859 when a thermoplastic polyester or polycarbonate is the thermoplastic resin and in U.S. Pat. No. 4,174,358 when a polyamide is the thermoplastic resin. As is well known, thermoplastic resin compositions within the scope of the disclosure of these two patents frequently can be classified as "super-tough" at room temperature, but the amount of modifying polymer required sometimes exceeds that allowable for a composition having good tensile modulus and acceptable flexural modulus and yield stress. Moreover, while certain of the compositions can be classified as "super-tough" at room temperature there is little or no improvement in the impact resistance at temperatures below room temperature. As used herein, the recitation "super-tough" means that a sample of the composition failed ductilely, as opposed to brittlely, at the temperature tested using the notched Izod toughness test (ASTM-D256) for 1/8 inch specimen.
In light of these prior art efforts failure to consistently produce super-tough compositions having good tensile modulus and yield stress as well as good low temperature impact properties, the need for an engineering thermoplastic composition having improved impact resistance even at lower temperatures and good flexural modulus at all temperatures and a method for preparing such an improved composition is believed to be readily apparent.